I use this method for all my HSTs.
If you just add 2 in to the squares for each size it is easy to figure. For 2 1/2 in triangles use 6 in squares, for 3 1/2 in use 8 in squares, etc. But since most rulers have an extra 1/2 in added I use them also it just wastes a little more fabric.
I also use Quilt In A Days triangle square up ruler that you only have to use one cut to square up. It is well worth the price.